Time and Work - General Aptitude

Q1:

A can do a piece of work in 4 hours; B and C together can do it in 3 hours, while A and C together can do it in 2 hours. How long will B alone take to do it?

A 8 hours

B 10 hours

C 12 hours

D 24 hours

ANS:C - 12 hours

A's 1 hour's work = 1 ;
4
(B + C)'s 1 hour's work = 1 ;
3
(A + C)'s 1 hour's work = 1 .
2
(A + B + C)'s 1 hour's work = ( 1 + 1 ) = 7 .
4 3 12
B's 1 hour's work = ( 7 - 1 ) = 1 .
12 2 12
Therefore B alone will take 12 hours to do the work.